1. Field of the Invention
The present invention relates in general to electronic timing systems and in particular to an apparatus and method for generating clock pulses from a power selsyn multiple motor drive system for a glassware forming machine.
2. Description of the Prior Art
The individual section (IS) glassware forming machine is well known and includes a plurality of sections, each having means for forming glassware articles in a timed predetermined sequence of steps. Typically, the sections are fed from a single source of molten glass. The source forms gobs of molten glass which are distributed to the individual sections. The sections are operated in synchronism at a relative phase difference such that one section is receiving a gob while another section is delivering a finished glassware article to a conveyor and one or more other sections are performing various ones of the intermediate forming steps.
The forming means in each individual section are typically operated by pneumatic motors or actuators. In early prior art machines, these pneumatic motors are controlled by a valve block which, in turn, is controlled by a timing drum. The timing drum for each section is driven from a line shaft which synchronizes all parts of the machine. Manually adjustable cams are positioned on the timing drum for actuating the valves in the valve block. Relative timing between the various forming means in each section can be adjusted by loosening, moving, and tightening the cams as the drum rotates.
Later prior art machines utilize an electronic control means to synchronize the operation of the individual sections. The electronic control means includes a master unit which is responsive to a clock pulse generator and to a reset pulse generator, both of which are driven by a line shaft. The master unit generates reset signals to a separate control circuit for each of the individual sections. Each control circuit includes a pulse counter responsive to the clock pulses and the reset pulses for counting the degrees of the section cycle. Each individual circuit includes forty-eight, three-decade thumbwheel switches for setting the degree of rotation of the machine thereon at which associated control signals are generated. Thus, each particular function of the glassware forming cycle is controlled by one of the thumbwheel switches. Such a control system is disclosed in U.S. Pat. No. 3,762,907.
Power selsyn multiple motor drive systems have been utilized in glassware forming machines to synchronize the gob feeder with the various forming means in each individual section. For example, U.S. Pat. No. 3,305,341 discloses a timing system for an IS glassware forming machine. A transmitter selsyn is driven by a motor which runs at a predetermined speed as it drives the gob feeder shears and gob scoop. A receiver selsyn has a rotor which is electrically connected to the rotor of the transmitter selsyn and is mechanically connected to drive the timing shaft and the take-out conveyor. In another type of glassware forming machine, the transmitter selsyn is mechanically driven by a motor which establishes the machine speed. The rotor of the transmitter selsyn is electrically connected to a first receiver selsyn which mechanically drives the timing shaft and the take-out conveyor and a second receiver selsyn which mechanically drives the gob feeder shears and gob scoop.
Since the selsyns are operated synchronously with respect to each other, it is desirable to generate clock signals to the electronic control means of the glassware forming machine based upon the synchronized speed of the receiver selsyns. To accomplish this, several types of electro-mechanical means, such as shaft encoders and proximity detectors, have been utilized to generate clock pulses at a rate proportional to the speed of the receiver selsyns. However, such electro-mechanical systems are unreliable, especially at low selsyn speeds, because variations in the amount of the load placed on the receiver rotors may cause false clock pulses to be generated.
U.S. Pat. Nos. 4,145,204 and 4,145,205 disclose timing pulse generators responsive to the frequency of the electrical power source connected to the drive motors for glassware forming machines. Such timing pulse generators require that the variable frequency of the power source be directly proportional to the rotor speed, such as in a power inverter drive system. However, when power selsyns are driven in the same direction of rotation as that of the primary magnetic field, as is the usual method of operation, the variable frequency of the power present in the rotor windings is inversely proportional to the speed of the rotors. Hence, the timing pulse generators of the prior art cannot be utilized on glassware forming machines operated by a power selsyn multiple motor drive system.